[sdiy] Better testing techiques for audio circuits

[Neil Johnson]

Justin,

As Harry said.

If you do want to go down the "audio quality" route there are four
typical methods:

1/ The nulling technique, where you apply a stimulus (e.g., a sine
wave) to a circuit and then measure the difference between the input
and output using a true RMS AC voltmeter.  This is the cheap method,
but done with care and attention to detail can produce good results.

2/ Low-distortion oscillator and notch filter.  Apply the low
distortion sine wave stimulus to the circuit under test, then take the
output through the notch filter to remove the source signal, leaving
distortion and noise measured with a true RMS AC voltmeter.  Tricky as
the notch filter needs to be very stable and properly tuned.

3/ Use an audio distortion analyser.  Expensive, but probably the
fastest method.  Top-of-the-range Audio Precision 2700 series.
Options on the used market are the HP 8903 family, or Tektronix,
Boonton, etc.

3/ Dynamic signal analyser (e.g. HP 3561A) can also measure THD and
noise, but again expensive, and not to the same level as a dedicated
distortion analyser.

It really depends on what level of quality you're trying to measure down to.

You could also use a PC soundcard and some suitable analysis software.
 Whether you believe it is another matter.

I wrote something along these lines on my test equipment page (scroll
down to the bit about the 8903B).

Cheers,
Neil
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